Municipal wastewater generally is piped from homes and businesses through a sewer piping system to a municipal wastewater treatment plant. In many towns and cities, storm water runoff infiltrates the sewer piping system. Municipal wastewater typically includes human urine and feces, food waste, household chemicals, and paper products such as toilet paper, tampons, and sanitary napkins. When mixed with storm water runoff, municipal wastewater can also include other litter and debris.
One of the purposes of treating municipal and sanitary wastewater is to allow sewage to be disposed without danger to human health or damage to the environment. Municipal wastewater is typically composed of approximately 99.8% to 99.9% water. The other 0.1% to 0.2% of municipal wastewater includes contaminants such as grit, fats and oils, oxygen-demanding substances, pathogens, plant nutrients, inorganic chemicals, and synthetic organic chemicals. One of the basic strategies of municipal and sanitary wastewater treatment is to remove or neutralize these contaminants and either dispose or reuse the water.
A typical municipal wastewater treatment process includes the following stages: pre-treatment, primary treatment, secondary treatment, tertiary treatment, and sludge treatment. The purpose of primary, secondary, and tertiary treatment is to remove and neutralize solids and pathogens so that the remaining water can be either reused or disposed of safely into the environment. The removed solids are known as sludge. The purpose of sludge treatment is to sufficiently dewater the sludge and neutralize the pathogens in the sludge so that the dewatered sludge can be safely recycled or disposed of.
Pre-treatment removes gross particulates and grit that can interfere with primary and secondary treatment processes. Primary treatment, also known as primary sedimentation, typically uses gravity to remove heavier-than-water solids, typically, in a basin or tank. The heavier-than-water solids settle to the bottom of the tank and are scraped and drained. The floating debris is skimmed from the top, passing over a weir. These solids are known as primary sludge.
Secondary treatment removes additional suspended solids and dissolved biodegradable material. This is typically accomplished using aerobic, or oxygen consuming, microorganisms to consume the soluble organic contaminants and other biodegradable material.
Tertiary treatment takes the product water from the secondary treatment and prepares it for reuse or for release into the environment. Tertiary treatment can remove remaining biological oxygen demand (BOD) dissolved solids, metals, pharmaceuticals, pesticides, and endocrine disruptors. There are a number of possible strategies for tertiary treatment. These can include media filtration, membrane filtration, ammonia and phosphate removal, and chemical disinfection.
The sludge can be biologically processed by either aerobic or anaerobic bacteria. The thickened sludge resulting from this operation typically has a solids concentration of 0.5% to 1% of solids to water. This sludge is further dewatered typically using a sludge filter press or a sludge-dewatering centrifuge. Both sludge filter presses and sludge-dewatering centrifuges require a minimum of 0.5% to 1% solids to water concentration for proper operation. The output of the dewatering process has approximately 15% to 35% solids to water concentration and typically is heat sterilized before disposal.